One example of a system for radio-location and time transfer is the NAVSTAR Global Positioning System (GPS) as described in the Global Positioning Service Signal Specification (2nd ed., 1995, USCG Navigation Center, Alexandria, Va.). Other examples include the GLONASS GPS maintained by the Russian Republic and the GALILEO system proposed in Europe. Typical uses for radio-location systems include airborne and oceanic navigation, although other uses for such systems are becoming increasingly common Ground-based systems such as networks for cellular telephony may also be used for radio-location and time transfer. Examples of terrestrial applications for radio-location technologies include asset tracking (for example, tracking of trucks and railcars), time transfer (for example, between fixed and mobile units of a cellular telephone network), locating cellular telephone users for emergency services (for example, as part of an “enhanced 911” initiative), and highway navigation assistance.
The NAVSTAR GPS includes a set of satellites or “space vehicles” (SVs) that transmit navigation messages on a 1.57542-GHz carrier (also called the L1 frequency). The navigation messages are transmitted at a data rate of 50 bits per second via a direct sequence spread spectrum (DSSS) signal that is BPSK (binary phase-shift-keying) modulated onto the carrier. To spread the signal, each SV uses a different one of a set of pseudo-random noise (PRN or PN) codes, which are also called “coarse acquisition” or C/A codes. Each C/A code has a chip rate of 1.023 MHz and a length of 1023 chips, such that the code repeats every one millisecond. The C/A codes are Gold codes which are selected for their autocorrelation properties. FIG. 1 shows a portion of the autocorrelation function of GPS PRN 1, which has a magnitude below 0.1 for all code offsets from +1 to +511 and from −1 to −511.
A NAVSTAR GPS SV may also transmit messages via a 10.23 MHz P(Y) code modulated onto a carrier at 1.22760 GHz (also called the L2 frequency). A GPS SV may transmit messages in a similar manner via several other carriers and/or codes as well. One common use of GPS signals is to support position location operations by terrestrial receivers. Typically, signals from at least four SVs are needed to resolve a position in three dimensions.
A GPS signal as received by a terrestrial user is exceedingly weak. For example, the received power of a GPS signal at the earth's surface is −130 dBm. In contrast, the thermal noise level is −111 dBm, or nearly 20 dB higher. A receiver inside a building may be expected to experience an additional 20 dB of signal attenuation from concrete and other building materials, such that a GPS signal received indoors may be about 40 dB below the thermal noise level. In these circumstances, an interfering signal well below the thermal noise level may be sufficient to prevent a GPS receiver from detecting a valid signal, despite the strong autocorrelation properties of the C/A codes.